1. Field of the Invention
This invention relates to arrangement for scanning a disc-shaped type carrier in a code-controlled printing device having a stepping motor which may be controlled in the forward and backward directions and which serves to set the type carrier.
2. Description of the Prior Art
A teleprinter is known which has a disc-shaped type carrier, the type carrier being set via a stepping motor which is controllable in the forward and backward directions. Rotation of the type carrier to the particular position to which the type carrier is to be set is always effected over the shortest path in dependence upon the last position occupied by the type carrier. For this purpose, an n-position electronic binary counter is provided which can be controlled by way of a series of stepping marks arranged on the periphery of the type carrier, or on the periphery of a pulse train disc, and the setting of the counter is compared, in each case, electronically with that of the code combination assigned to the type which is to be set. Such a system is disclosed by Martin Ludwig in his U.S. Pat. No. 3,816,656, granted June 11, 1974 and assigned to the same assignee as the present invention. By this reference, the disclosure of this patent is fully incorporated herein.
However, in such an arrangement very high requirements are imposed on the synchronization between the type carrier and the binary counter. To ensure this synchronization, it is known to arrange, either on the type carrier or on the pulse train disc, an additional scanning mark which is scanned once per rotation. The pulse obtained in this manner then serves to reset and to synchronize the binary counter.
However, frequently it is insufficient to obtain a synchronization pulse only once per rotation. Therefore, it has already been described in the aforementioned Ludwig patent, to provide a synchronizing mark which occurs once on the type carrier periphery, or on the pulse train disc periphery, this mark being assigned to a specific character which occurs frequently in the text, preferably to the character E, and to synchronize the binary counter independently of the usual setting to the code signal assigned to this predetermined character. One thus obtains, via a scanning element which the mark passes with relative frequency, a very large number of synchronizing pulses. Therefore, the synchronization between the setting of the type carrier and the binary counter is practically always ensured.
The assignment of the scanning mark to a specific character does mean, however, that the scanning mark itself is only the same size as an element of the type carrier which bears the type characters. If, for example, 60 type elements are distributed over the periphery of the type carrier, only one sector of approximately 6.degree. is available for the scanning mark. In order to safely recognize such a scanning mark, a high-resolution scanning device is required to recognize the scanning mark even when it moves past the scanning point at high speed. Such scanning devices are primarily in the form of expensive, elaborate opto-electronic scanning elements.